(1) Field of the Invention
The present invention relates to a cutting tool comprising a sintered body layer containing high density boron nitride or diamond and a sintered cermet layer.
(2) Description of the Prior Art
There are two kinds in the high density boron nitride, the one is cubic system boron nitride and the other is wurtzite-structured boron nitride. While, there are two kinds of the diamond, the one is cubic system diamond and the other is hexagonal system diamond. All of these high density boron nitrides and diamonds must be treated under high pressure and high temperature in order to sinter them. Further, high density boron nitride and diamond are expensive as a raw material, and therefore when they are formed into a sintered body and used as a cutting tool, the resulting cutting tool is very expensive as compared with conventional cutting tools. Accordingly, in order to lower the raw material cost occupied in the production cost of cutting tool, a thin layer having a thickness of 0.5-2 mm of sintered body containing high density boron nitride or diamond has been generally laminated on a cemented carbide layer. Substantially all of the cutting tools sold in the market use a sintered body layer laminated on a cemented carbide layer. The use of the sintered body (hereinafter, referred to as laminated sintered body), which is formed by laminating a sintered body layer containing high density boron nitride or diamond on a cemented carbide layer, has the following merits that the use amount of the expensive raw materials can be saved, that the hard but brittle sintered body layer containing high density boron nitride or diamond can be reinforced by the lining of the cemented carbide layer, and that, although the sintered body layer containing high density boron nitride or diamond, can not be directly brazed to a shank of tool, the surface of the cemented carbide layer lined to the sintered body layer can be brazed to a shank of tool or to a base having a shape of throwaway tip.
However, the laminated sintered body having the above described merits has the following drawbacks at the same time. That is, the sintered body layer portion containing high density boron nitride or diamond in the laminated sintered body is thin, and therefore it is very difficult or substantially impossible to engrave the surface of the sintered body layer so as to form a chip breaker or to work so as to form a projected chip breaker. As a means for obviating the drawbacks, Japanese Patent Laid Open Application No. 140,284/79 discloses a method, wherein a plate-shaped element is bonded to the rake face of a sintered body layer containing high density boron nitride or diamond or a mixture thereof, and the plate-shaped element is left on the rake face and used as a chip breaker. However, the plate-shaped element bonded to the rake face in this method is made of cemented carbide, molybdenum, steel or the like having a toughness higher than that of the sintered body layer, and therefore the plate-shaped element is lower in the hardness than the sintered body layer containing high density boron nitride or diamond, and is easily worn under a severe cutting condition. Furthermore, conventional laminated sintered body can not be used in a cutting tool, which uses the side surface of throwaway tip as a rake face, in so far as the laminated sintered body has the conventional shape.
The inventors have taken the above described problems into consideration, and made various investigations in order to save the use amount of expensive raw material and concurrently to solve the above described drawbacks, and accomplished the present invention.